Lithium-ion secondary batteries with high energy densities have been used as power sources for portable electronics such as a mobile phone and a notebook computer.
An electrode member of a lithium-ion secondary battery includes a positive electrode plate, a separator, and a negative electrode plate. Regarding a positive electrode material, an aluminum alloy foil has been used as a support, having excellent electrical conductivity and less heat generation without affecting electrical efficiency of a secondary battery. An active material having a lithium-containing metal oxide such as LiCoO2 as a chief component is applied on a surface of the aluminum alloy foil. Its production process includes: applying an active material with a thickness of about 100 μm on both sides of an aluminum alloy foil with a thickness of about 20 μm; and drying the active material to remove a solvent therefrom. Further, in order to increase the density of the active material, compression forming is performed with a pressing machine (hereinafter, this step of compression forming performed with a pressing machine is referred to as press working). The positive electrode plate as so manufactured, a separator, and a negative electrode plate are stacked, and then the resulting stack is wound. After a shaping process is performed so as to encase the stack, it is encased.
An aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery has several problems that cuts occur during application of an active material and that ruptures occur at a bending portion during winding. Thus, a higher strength is required. At a drying step after the application of the active material (hereinafter referred to as “drying step”), heat treatment is carried out at about 100 to 180° C. Accordingly, a lower strength after the drying step is likely to generate middle waviness during press working. This induces wrinkles during winding, which reduces adhesion between the active material and the aluminum alloy foil. Besides, a rupture is likely to occur during a slitting process. When the adhesion between the active material and a surface of the aluminum alloy foil decreases, their detachment is facilitated during repeated operation of discharge and charge. Unfortunately, this causes its battery capacity to decrease.
Recently, a high electrical conductivity has also been required for an aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery. What is meant by the electrical conductivity refers to physical property indicating how easily electricity is conducted in a substance. The higher the electrical conductivity is, the more easily the electricity is conducted. Lithium-ion secondary batteries used for automobiles and/or electric tools necessitate a higher output characteristic than lithium-ion secondary batteries used for consumer-use mobile phones and/or notebook computers. When a large current flows, a lower electrical conductivity causes internal resistance of a battery to increase. Consequently, this reduces its output voltage.
3003 Alloy is generally used as the aluminum alloy foil for lithium ion secondary batteries having high strength. 3003 Alloy is characterized by its high strength due to the addition of elements such as Si, Fe, Mn, Cu and the like. In particular, since Mn forms solid solution and precipitates finely, decrease in strength during heat treatment is small. However, since Mn solid solution decreases conductivity, the conductivity of 3003 alloy is extremely low when compared with the conductivity of an aluminum alloy having aluminum purity of 99% or higher. In conclusion, 3003 alloy has difficulty in satisfying both of the high strength and the high conductivity that are required in the aluminum alloy foil for lithium ion secondary batteries.
Patent Literature 1 discloses an aluminum alloy foil for battery current collector having a tensile strength of 98 MPa or higher. Patent Literature 2 discloses an aluminum alloy foil for an electrode current collector of lithium ion secondary batteries having a tensile strength of 200 MPa or higher. However, Patent Literature 1 and Patent Literature 2 both do not mention of the conductivity.
Patent Literature 3 discloses a method for preventing plastic deformation and for preventing the detachment of the active material during the press working, by enhancing the strength of the aluminum alloy foil. However, the aluminum alloy foil disclosed is an alloy added with Mn, Cu, Mg as the main element, and thus cannot satisfy the required high electrical conductivity.
Patent Literature 4 discloses an aluminum alloy sheet, the solid solution content of Fe being less than 50 ppm, the thickness of the sheet being 0.1 to 2 mm, and the tensile strength being 145 to 200 MPa. However, the thickness of the sheet being such prohibits the application as an electrode current collector. Further, since the solid solution content of Fe is low, the strength of the alloy extremely decreases when heat treatment at 120 to 160° C. is performed for 15 minutes to 24 hours.